Re: [uml-devel] [patch] host-skas3 for 2.6.9-rc2

[BlaisorBlade <blaisorblade_spam@yahoo.it>]

On Monday 20 September 2004 17:37, Gerd Knorr wrote:
> > Without the SKAS patch, init_new_context does not call
> > arch_pick_mmap_layout(), so it should not after ( and init_new_context
> > calls, actually, init_new_empty_context()). I think that the proper place
> > to add the call is open_proc_mm(). Do you agree on this?
>
> Sounds ok, changed that, tested, works, new version below.
Yes, agreed.
> There is 
> also another fix in enter_lazy_tlb()
In switch_mm, actually.
> : use the per_cpu() macro (also 
> new in 2.6.9-rc I think).
Ah, yes, that is the only change which I had made when porting the SKAS patch 
to 2.6.9, while fixing the reject below. Basically, what we do is to make 
sure that the BUG_ON check that we remove cannot be triggered; that check is 
done if prev == next, so we must make sure to give to "prev" the value that 
"next" is supposed to have.

The reason is that with PTRACE_SWITCH_MM a thread can be scheduled away while 
having one MM and rescheduled while having a new MM; and if when it's 
scheduled away, a kernel thread goes executing, the kernel thread (using a 
lazy TLB) will keep using the old MM, the one switched away with 
PTRACE_SWITCH_MM, and the one you get with per_cpu(...). The "prev" and 
"next" args, instead, come from current->mm, so prev is wrong (the kernel 
code didn't notice that current->mm has changed). However, this can only be 
triggered on SMP, when the ptracing thread (the UML kernel thread) executes 
on a different CPU than the userspace thread.

Bye
-- 
Paolo Giarrusso, aka Blaisorblade
Linux registered user n. 292729


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